Earplugs are commonly worn by workers who are subjected to loud noise, to protect their hearing. Occasionally, an earplug may fall from a worker, as when he/she is handling it as to pull it out or push it into the ear canal. In contamination-sensitive processes such as in the preparation of food or medicine, steps are taken to minimize the possibility that an earplug will fall into the material being produced. This often involves tying together a pair of earplugs with a cord. The cord may limit falling of one of the earplugs as where the other earplug is held in the worker's ear or the cord is separately held. In addition, an earplug that falls into a batch of food or medicine must be detected.
Earplugs and cords can be detected by attaching a metal fitting to the earplug or cord for detection by a metal detector. It is usually preferred that the metal be detectable by a magnetic detector, and an iron based alloy is usually preferred because it is of low cost and sensitive to detection by a magnetic detector. Magnetic detectors are often more sensitive than metal detectors. Applicant notes that certain ceramics (which are as hard as hard metals) are magnetically detectable and are the equivalent of metal for the purposes of the invention. Both the earplug and any cord that connects them should be separately detectable because an earplug may become detached from a cord. U.S. Pat. No. 4,936,411 describes the placement of a metal ball of about 2 millimeters diameter (and 6 cubic millimeters volume) in a deep passage in the rear of an earplug and the placement of a metal ring or crimp barrel of about 2 millimeters diameter and 8 millimeters length (volume of about 2 cubic millimeters) around a cord, to detect either one by magnetic, electrical or x-ray detectors. U.S. Pat. No. 5,727,566 describes a metal ring of about 0.2 inch diameter (5 millimeters diameter and a volume of about 5 cubic millimeters) attached to an earplug for detection.
Although the presence of a metal ball or ring greatly helps detect an earplug or cord, there is still a possibility that the earplug or cord will go undetected. If undetected in material being processed, a metal ball or ring of about 2 millimeters diameter may be even more disastrous than a solely polymer earplug or cord. Such metal piece may do more damage to a person eating the metal piece or to machinery that processes the material, than a solely polymer earplug or cord.
It would be desirable if a corded earplug pair comprising a pair of earplugs and a cord with opposite ends attached to the earplugs, were constructed so that all components of the combination could be detected by a metal and/or magnetic detector. It would be especially desirable if the earplugs and/or cord were separately detectable by metal and magnetic detectors, without the earplugs or cord containing a metal piece of appreciable diameter or volume (more than 0.1 cubic millimeter).